The need for data storage capacity is increasing rapidly every year. Today, a company's storage system may be distributed over different locations and comprise multiple server racks in one or multiple data centres where each rack houses multiple storage servers. Some companies outsource their storage needs to external storage providers offering cloud based storage solutions.
At some point in time, a storage system user may decide to migrate his data from its current storage system to a new one. This decision may be driven by several factors. A first factor may be financial considerations, where the new storage system provider offers the same or more capacity for a better price. Another factor may be that the capacity of the current storage system can no longer be increased and a migration to a new and larger storage system is inevitable.
In all of these cases, a data migration is to be performed, i.e., all data on the source system needs to be copied to the destination system and, at some point in time, users need to be switched to the new destination system. During the actual switch or cutover, the users are typically denied access to both storage systems in order to ensure data integrity. This way users cannot write to data that is being copied which could cause data corruption or users cannot write to a data location that has already passed the migration which could cause data loss.
For large storage systems serving tens of Terabytes up to several Petabytes of data, a single copy of all data may take in the order of days, weeks or even months. Denying user access to the storage system for such a long time is simply unacceptable and thus solutions are needed to shorten the switchover or cutover time.
WO12149220A discloses the concept of incremental copies to shorten the actual cutover time. First, an initial or baseline copy is made of all data to be migrated from the source to the destination system. Then, one or more incremental copies are made before the actual switchover. An incremental copy only considers the differences between the source and destination system. It thus applies all changes from the users that are still using the source storage system. During the initial copy and incremental copies, the users are still allowed access to their data on the source storage system. Then, at a certain planned point in time, the actual cutover is performed. During the cutover, the users are denied all access from the storage systems and a last or cutover incremental copy is made. When the cutover copy is done, the users are switched to the new destination storage system and can again access their data.
Although the above concept greatly reduces the actual cutover time, it is still an object to further shorten the cutover time.